1. Field
This disclosure pertains to regulators for power supplies and, more particularly, to regulators for isolated flyback converters.
2. Description of Related Art
Power supplies used in telecommunications, health care, industry, and other applications may require electrical isolation between the input to the supply and the output from the supply. A transformer is often used to provide this isolation.
The transformer may be placed in a configuration known as a flyback converter. A flyback converter often regulates the output of the power supply by controlling a series of pulses that are delivered to the primary winding of the transformer. When the output needs to be increased, the on-time of the pulses may be increased. Conversely, when the output needs to be decreased, the on-time of the pulses may be reduced.
The output of the power supply is usually sensed to determine how the pulses need to be altered to achieve a desired value. When complete electrical isolation must be maintained between the input and the output of the power supply, this sensing may also need to be done in an electrically-isolated manner.
To achieve this electrical isolation, an opto-isolator is sometimes used to relay the output level back to the input control system. The feedback in this configuration utilizes an optical path over which electrons cannot travel. Opto-isolators, however, can increase the size of the power supply, increase costs, and may limit the ability of the power supply to make needed adjustments quickly.
Another technique is to monitor the voltage on the primary winding of the transformer in the flyback converter. This is commonly referred to as “primary side sensing.” This monitoring has typically been done during off periods of the pulses when the primary winding has been disconnected from the supply voltage and current is flowing through the secondary winding of the transformer. This technique works due to inherent characteristics of the transformer. These inherent characteristics cause the voltage across the primary winding during this period to be approximately equal to the output voltage of the power supply, times the ratio of turns in the primary winding to the turns in the secondary winding.
The output regulation that is accomplished using primary side sensing, however, may not be adequate. For example, the regulator may not be able to maintain the output voltage at a constant level when the load on the supply gets too low. To avoid this potential problem, isolated flyback converters are sometimes designed so that they always draw a minimum amount of current. This can be wasteful of energy.